Berlin Sugar Symposium

Due to increasing energy costs and environmental regulations, process optimization and production cost minimization becomes a major focus of interest in the food industry. This presentation deals with the collection of production-relevant data in the laboratory in order to calculate the operating costs of e.g., Lewatit® ion exchange systems for purification of sugar and non-sugar solutions. Based on laboratory date it is therefore possible for many different sugar and non-sugar solutions that can be refined with ion exchange resins to calculate the chemical and water consumption for new plants or to optimize them for existing plants. The presented LewaPlus® Food module is a tool designed for the food industry, which enables the calculation of new and existing desalination plants. The calculation of process costs is possible via the Food module as well.

Non-sucrose sugars (such as polyols) have nutritional benefits due to their low calorie content and can be used as a suitable alternative to traditional sugar in certain products (such as baked goods and chocolate). These sweeteners are primarily sold as highly concentrated syrups. Recently, the pharmaceutical, cosmetic and chemical industries have also shown interest in these substances. In addition to high purity products, there is a growing demand for crystalline or solid materials in place of syrups. Various processes such as crystallization or spray drying are available. For optimal process design, the final quality requirements, which often depend on the application (such as tableting), as well as the physical and chemical properties, such as solubility or stability under thermal stress or in basic and acidic environments, of non-sucrose sugars must be taken into account.

Crystallization of sucrose is not a challenge due to the extensive industrial experience. However, this experience cannot be directly applied to non-sucrose sugars. Producers are challenged by differences in solubility, polymorphism, thermal instability or high viscosity. For some non-sucrose sugars, very little literature is available beyond solubility specifications at 20°C. Experience with the crystallization of these substances is a competitive advantage and is well protected by the companies.

The lecture presents own results on the physical and chemical properties of polyols as well as an overview of patents and other research work. Information relevant to process control will be provided. For example, unadapted dry substance measurements can lead to erroneous results and suboptimal process control. Data on stability versus pH and temperature for various non-sucrose sugars are presented. An example shows that different nucleation behavior can lead to spontaneous crystallization of the entire solution by adding seed crystals. Using literature data, the problem of polymorphism under unfavorable process control is addressed. The final crystal polymorph determines the physical and chemical properties and thus the final applicability. The contribution is intended to draw attention to the problems in the crystallization of non-sucrose sugars, to raise awareness of certain issues, to provide solutions and alternatives, and to stimulate discussion.

The optimization of the crystallization process requires the technical knowledge of the sucrose solubility in syrup. Therefore, the study of sucrose solubility is a key factor to control the nucleation, and strongly implies on the industrial yield, thus maximizing the production gain by reducing the recirculation of valued processed product. Although the sucrose solubility in pure water solution is widely known, the sucrose solubility in non-pure solutions from cane sugar still requires research and knowledge. Despite early studies are dated 1928, they’ve never been improved in quality or accuracy. Some of technical papers, which were found, present the evaluation of sucrose solubility in water in the presence of impurities, but remain at higher purities close to 100%. This work studies the sucrose solubility in impure water solutions, which are produced with the final molasses from the processing of the cane sugar, establishing a relationship between sucrose solubility and the purity of the solution, at previously defined temperatures. The analysis in this work were conducted at 40 °C, at 50 °C and at 60 °C, following the Wagnerowski method or “polish test”, with different purities of solution, varying from 99% to 45% and were obtained through the saturation of samples with excess sucrose. The Vavrinecz equation for the calculation of the saturation coefficient was tested and validated. Measurements have been taken with refractometry, polarization and high-performance liquid chromatography. The knowledge achieved of the sucrose solubility in technical (impure) solutions, allows the determination of the solution seeding point concentration, which corresponds to its purity and process temperature, to ensure the desired optimal crystal development in the industrial process. The results achieved confirm to the cane sugar, the already studied beet sugar trends.

The European sugar industry plans to produce carbon-neutral by the year 2050. The crystallization in the sugar house is one of the major energy-consuming processes within a sugar factory. Combined heat and power generation as well as multiple stage evaporation are measurements to reduce the overall energy consumption. Consequently, the temperature difference between heating steam and massecuite in the calandria of discontinuous evaporative crystallizers is reduced. Vapor nucleation within the calandria is deemed to be impossible due to the increasing massecuite level towards the end of a strike. The ensuing risk of undesired nucleation due to decreased evaporation activities and, hence, increased inhomogeneities is an issue that needs to be tackled and requires in-depth understanding of the processes in the crystallizer.

The present study investigates the role of sucrose crystals within the evaporation part of the main crystallization process. A numerical model is developed to predict the evaporation rate of a discontinuous evaporative crystallizer based on analytical models of the underlaying microprocesses such as vapor bubble nucleation and growth. Various system parameters such as heating steam and pan pressure dictate the kinetics of evaporation. In agreement with theoretical considerations and recent laboratory studies, the surface of the sucrose crystals distributed within the suspension is assumed to serve as a valid nucleation source. The contribution of crystal-based evaporation is compared to the overall evaporation rate at several points in time of the semi-batch process.

The model is able to reproduce evaporation rates determined in industrial crystallizers. Considering wall-nucleation exclusively, industrial observations cannot be explained at all stages of the process. The importance of crystal-based evaporation increases with the progress of a strike due to the decreasing amount of vapor bubbles nucleated in the calandria. Apart from being the product, sucrose crystals seem to serve as a major factor to ensure sufficient evaporation activities in the crystallizer.

Over the last number of years, the Tereos R&D group developed tools for evaluation of massecuite during and at the end of a crystallisation strike to enable comparison and optimization of the performance of individual pans. 

The basic procedure uses standard laboratory equipment that can be used by factory personnel with relative ease and could be optimized to be representative of the massecuite in the pan.

In most factories the results were surprising and often staggering.  For example, if a factory operates 5 identical white pans it is assumed that the massecuite from these pans would be similar if all other parameters are optimized.  This is not always the case and can at worst case have dire consequences. The only way to know that there is actually a problem is to look at the massecuites from individual pans. 

The study at different factories enabled simple optimization of individual as well as battery of pans without any capital expenditure.  Results included significant energy reduction, optimization of pan operations and sugar quality improvements.

Especially in the last campaign, rotten beets and dextran were present in European sugar factories. Many studies have investigated the influence of dextran and the application of dextranase on filtration and crystal morphology. In addition to filtration problems, rotten beets are known to have higher juice and sugar colors. Higher color transfer factors are associated with poor juice quality, although the actual causes are unknown. Dextran, invert sugar and raffinose levels are higher in poor quality beets. However, studies on their influence on color incorporation are underrepresented. Therefore, in the present study, the influence of dextran alone or dextran in combination with invert sugar and/or raffinose on color incorporation into the sucrose crystal was investigated. For this purpose, crystallization tests were carried out in duplicate in three different work packages:

1. Thick juice with the addition of different concentrations of dextran (0, 1000, 3000, 5000 ppm).
2. Pure sugar solution with a synthetic melanoidin, dextran, glucose and raffinose as a full-factorial design.
3. Pure sugar solution with a synthetic melanoidin, dextran (3000ppm), and dextranase (0, 4, 8 ppm).

The results of this work show that the crystal growth rate changes significantly in the presence of dextran or a combination of dextran with invert sugar and/or raffinose. Degradation of dextran by dextranase reduces this effect. It was also found that the color value of sucrose crystals increased significantly with increasing dextran concentration. The color comparison between the affinated and non-affinated samples showed that the attachment of colorants to the crystal surface and the incorporation of the colored compounds into the crystal in the presence of dextran are subject to different mechanisms than in the case of invert sugar and raffinose. For example, in the presence of dextran, the color is incorporated into the crystal to a greater extent. Dextran degradation has a significant effect on crystal color, even low dextranase concentrations of 4 mg/kg have positive effects.

Photon Density Wave (PDW) spectroscopy is a fiber-optical method for the determination of the optical properties of highly light scattering systems, i.e. the absorption and the reduced scattering coefficient (µa and µs’, respectively). The absorption coefficient is influenced by the chemical composition at the chosen measurement wavelength. The reduced scattering coefficient is dependent, amongst others, on the size and number of scattering particles. PDW spectroscopy allows the calibration-free and probe-based in-line characterization of highly concentrated liquid dispersions.

First experiments have shown that it is a suitable tool for monitoring the sucrose crystallization at industrially relevant concentrations. Furthermore, this technology is very robust against probe fouling which is highly advantageous for monitoring crystallization processes.